a. Field of Invention
This invention pertains to a device and method for displaying various messages on an electronic display unit comprising a plurality of elements arranged in a two dimensional array.
b. Description of the Prior Art
Display devices having a two dimensional array of display elements are frequently used for displaying various types of information. Typically such display units are relatively large so that they are visible from various distances, and the messages are displayed by selectively activating and deactivating the display elements to generate various alphanumeric or other types of characters. Furthermore it was found advantageous to move messages vertically or horizontally across the display unit by activating preselected display units in a preselected sequence. Normally the display elements are activated by the so-called scanning array technique in which the display elements are activated sequentially. However it was found that due to the time lag between the activation of first and last display elements of a particular character, the message often appears to the viewer to tilt in the direction of movement of the message. Furthermore, because the display elements are activated sequentially, the whole message appears to waver.
Some of the display elements comprise means of emitting light such as light emitting diodes. The intensity of these type of elements is typically varied by turning them on and off periodically and changing their duty cycle. However it was found that due to slight manufacturing differences there is a difference in intensity between the elements so that the overall message is not uniform. Furthermore it was found that turning these elements on and off causes the message to flicker.
In general it was found that a message displayed by the scanning array technique is difficult to see while a person is moving.